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The methods of storing and delivering water will vary with its source. In rural districts, deep wells and springs are the best sources of supply; but in large towns they are found to be insufficient for the 송탄오피 wants of a rapidly-increasing population; and they can only be multiplied in a given district within certain limits, as every well drains a large surrounding area. The supply from surface wells in gravel or sand beds or in chalk districts is liable to fail in seasons of drought; but deep wells in oolite or chalk formations, and in the new red sandstone, generally yield a constant and abundant supply.

When the water is supplied from upland surfaces, springs, or small streams, a collecting reservoir is required. This is generally a natural valley below the level of the source of supply, but of sufficient elevation above the place supplied to allow the water to be distributed by gravity, without any pumping apparatus. The reservoir should be large enough to hold five or six months’ supply, and its embankment should be perfectly water-tight, and of great strength.

When water is collected from upland surfaces, it is important to know the amount of rainfall to be reckoned on. If we know the area of the surface which drains towards the reservoir, and the average rainfall, the total rainfall is easily calculated. This will, however, differ greatly from the available rainfall, owing to the losses from penetration into the ground, evaporation, and other causes. The amount lost will vary, according to the season, from one-half to seven-eighths of the total rainfall; and according to the soil (page 68). The proportion of percolation in the chief water-bearing strata surrounding London varies from 48 to 60 per cent. (Prestwich). It is less when the ground is steep and the rainfall rapid, and usually less in winter than in summer.

Water collected near its actual place of fall, and from uncultivated districts, is always purer than that collected further from its source, and from cultivated land.

From the collecting or impounding reservoir, water is carried by the aqueduct or conduit either directly into the service-pipes, or when the pressure is too great, into a second service-reservoir, resembling the impounding reservoir in general structure, and capable of holding a few days’ supply.

This must be high ground, above the level of the highest75 houses to which water has to be supplied, as water cannot rise above its own level. When this cannot be arranged, the water is pumped into tanks at a higher level, and distributed from them.

The greatest hourly demand for water being double the average hourly demand, the water-mains supplying a town must have double the discharging power that would be required, supposing the demand was uniform. The first requisite of a supply of water is that it should be abundant, and sufficient in amount for any extra strain on its capacities. Water ought to be laid on to every house, and to at least two floors of the house. Anything preventing free access to water, militates against cleanliness.

Cast-iron is the most serviceable material used in the construction of the main water-pipes; it is coated with pitch, or Dr. Angus Smith’s varnish, or with magnetic oxide of iron (Barff). The service-pipes to each house are generally made of lead, and the ease with which this material can be bent and curved, and carried to the different floors of a house, makes its use very convenient. Lead pipes, furthermore, can be easily obliterated in case of bursting, and so any waste of water and flooding of the house minimised. Some kinds of water, unfortunately, act on and dissolve lead; this is especially true of soft waters and those containing organic matter. Shallow wells, being very liable to organic pollution, ought never to have the supply-pipe of their pumps made of lead. With hard waters, lead pipes may generally be used safely. When the quality of the water renders lead pipes objectionable, the use of iron, tin, zinc, tinned copper, earthenware, gutta-percha, and other materials, has been suggested. Of these, cast and wrought-iron pipes are the most serviceable, or pipes composed of an inner lining of block-tin and an outer of lead, a layer of asbestos intervening to prevent galvanic action between the metals. According to Rawlinson, “supply-pipes of wrought-iron are cheaper, stronger, and more easily fitted than service-pipes of lead;” but it is urged against them by Perry, that with soft water they become choked by rust in a few years. If galvanized they are more durable. Cast-iron pipes are rusted less easily than wrought-iron.

When the water-supply is from a river, filtering beds are needed, in addition to the parts of a water-service hitherto described. Moreover, since the river is usually at a low level, the water, after passing through the filtering beds, requires to be pumped into raised tanks, from which it is delivered.

In laying down water-pipes, in the streets and to houses, it is very important to make the 송탄오피 distance between them and all drains and gas-pipes as great as possible. Suction of gases or liquids may occur into leaky pipes, even though these contain water, and still more when they are empty; and disease has occasionally been traced to this source. Thus if sewers and water-mains are laid in the same trench, foul matters which have escaped into the soil from the former may be sucked into the latter. This may happen76 if the water-mains are leaky, even when they are running full. Experiments have shewn that the flow of water causes a partial vacuum and insuction at the defective points. During intermissions of supply when the mains are partially or entirely empty, the danger of leakage into them is still greater. Coal-gas has been similarly sucked into water-mains.

The pipes bringing the water to a house may be kept constantly filled with water, or only for a limited time once or twice a day. The intermittent system of supply necessitates the provision of cisterns or water-tanks, in which water can be stored in the intervals of flow of water. With a sufficient and properly-distributed public supply of water, no cistern ought to be required.

Cisterns.—Cisterns for the supply of potable water may be made of iron, slate, stone, glass, glazed earthenware, or brick lined with Portland cement. Other materials have been used, as timber, lead, and zinc. Timber is inadvisable, as it easily rots; lead is very objectionable, owing to the possible solvent action of the water on it. Zinc or galvanized iron cisterns are also acted on by soft water; but they may be used with most waters. Galvanized iron is iron coated with a thin layer of zinc. Iron cisterns soon rust; but this may be prevented by giving them a coating of boiled linseed oil before they leave the foundry. Stone cisterns are too heavy for use, except in basements. Slate cisterns are good, but are apt to leak; the points of leakage have occasionally been stopped with red lead, which is attacked by the water, and thus lead poisoning results. If the slate is set in good cement (not mortar, as this makes the water hard), it is a good material for a cistern.

Every cistern should have a well-fitting lid, always kept closed, to avoid the entrance of dust of various kinds, or even dead cats, birds, etc. Noxious gases may be absorbed by the stagnant water.

The cistern should be easy of access. If it is indoors, the cistern room should be well ventilated; and in any case the cistern should be periodically visited and cleaned out. When the cistern is full, a ball-tap prevents any further flow of water; and if this does not act properly, an overflow pipe carries off the excess of water.

Cisterns badly arranged or neglected have been in the past a common source of disease. (1) The overflow pipe should not pass into any part of the water-closet apparatus or the soil-pipe, or into the supply pipe to the water-closet.

Where the overflow-pipe discharges into the soil-pipe or closet pan, foul gases or even solid particles may find their way into the cistern.

(2) No water-closet ought to be supplied from the same cistern as supplies drinking water, as the pipe leading down to the closet may when the cistern is accidentally empty carry noxious effluvia into the cistern. A separate flushing cistern capable of discharging two to three gallons of water should be provided for each closet.

With a constant supply of water, cisterns are only required for water-closets and for hot-water apparatus (see pages 168 and 164).

Constant and Intermittent Services.—With an intermittent service of water, during the intervals of supply, water is77 only obtainable from cisterns, water-butts, etc. The objections against this system are that—(1) The cisterns required 송탄오피 are expensive, and liable to get out of order and become foul. (2) Their overflow pipes may improperly communicate with the soil pipe or with some other part of the drainage, instead of opening into the external air. (3) Putrid gases, from neighbouring ventilating-pipes or other parts of the drainage system, are liable to be absorbed by the stagnant water in the cistern. (4) The chief objection to an intermittent supply is that, during the intervals in which the water-mains are empty, foul air and liquids from the contiguous soil and drains are liable to be sucked through imperfect joints into the pipes. (5) In case of fire, the supply of water in the system is insufficient. In certain towns rates of insurance against fire have been reduced on replacing an intermittent by a constant service of water.

On the other hand it is urged that more expensive fittings are required for a constant service; and that, when taps are left open or pipes burst, the waste of water is much greater than with a cistern supply. The balance is decidedly in favour of a constant supply without storage cisterns. Where storage cisterns are in use, the taps for drinking-water should be connected with the “rising-main,” before it supplies the cistern.

The Advantages of the Constant Service may be thus summarised:—

(1) Owing to the absence of cisterns, the risks connected with stagnant water, and with improper arrangement of overflow pipes, are obviated.

(2) The risk of suction into supply mains of external contaminations is reduced to a minimum, since the pipes are never empty.